Sensing three dimensional (3D) scene structure is an integral part of applications ranging from 3D microscopy to geographical surveying. While two dimensional (2D) imaging is a mature technology, 3D acquisition techniques have room for significant improvements in spatial resolution, range accuracy, and cost effectiveness. Humans use both monocular cues, such as motion parallax, and binocular cues, such as stereo disparity, to perceive depth, but camera-based stereo vision techniques suffer from poor range resolution and high sensitivity to noise. Computer vision techniques (including structured-light scanning, depth-from-focus, depth-from-shape, and depth-from-motion) are computation intensive, and the range output from these methods is highly prone to errors from miscalibration, absence of sufficient scene texture, and low signal-to-noise ratio (SNR).
In comparison, active range acquisition systems, such as light detection and ranging (LIDAR) systems and time of flight (TOF) cameras, are more robust against noise, work in real-time at video frame rates, and acquire range information from a single viewpoint with little dependence on scene reflectance or texture. Both LIDAR and TOF cameras operate by measuring the time elapsed between transmitting a pulse and sensing a reflection from the scene. LIDAR systems consist of a pulsed illumination source such as a laser, a mechanical 2D laser scanning unit, and a single time-resolved photodetector or avalanche photodiode. A TOF camera illumination unit is composed of an array of omnidirectional, modulated, infrared light emitting diodes (LEDs). The reflected light from the scene, with time delay proportional to distance, is focused at a 2D array of TOF range sensing pixels.
A major shortcoming of LIDAR systems and TOF cameras is low spatial resolution, or the inability to resolve sharp spatial features in the scene. For real-time operability, LIDAR devices have low 2D scanning resolution. Similarly, due to limitations in the 2D TOF sensor array fabrication process and readout rates, the number of pixels in commercially-available TOF camera sensors is currently limited to a maximum of 320×240 pixels. Consequently, it is desirable to develop novel, real-time range sensors that possess high spatial resolution without increasing the device cost and complexity.